Our home is 2x4 construction one story 6/12 pitch. The plan is to add a 12x28 room in the attic which is above three BRs, hallway, and 2 baths. A 12 x 28 br and closet, 12x28 hallway bath and br, and a 16x28 mr br bath & closet. This part of the house is 28 X 40 above a crawl space. The other part of the house sets on a basement. I will be adding 28' lvl beams in the crawl space under the the walls of the 12x28 hallway bath and br which is approx in the center blocked solid to the foundation. The attic room will have a 5' knee wall on the south and a 7' knee wall on the north. The west end of the room will have a window and the east end will have a door to the stairs. The stairs to the attic will be above the stairs to the basement. I plan to use 2x10 12' floor joists 16" oc and build a frame out of 2x4 for the knee wall and 2x8 up the slant (room for insulation and air to roof) of the roof to the collar ties. Not sure about height of ceiling. The roof is supported by 2x4 40' double fink trusses 24" oc. I have been advised to laminate each truss with 4x8' 1/2" plywood and a truss manufacture indicated that building a frame with 2x10 floor joists with 2x4 & 2x8 framing was enough. Any comments or suggestions would be appreciated.

How big are the lvl beams, how heavy are they and do you have a plan for getting them in place?
How high is the foundation walls from footing to sill plate?
You are replacing the trusses with rafters and ceiling joists, full length rafter? Could be tricky to get in.
2x10 rafters is the standard today, increases the insulation value by 30%
How high is the seat cut of the trusses? The seat cut is the distance between the top outside of the outside wall and the bottom of the roof sheeting.
The trusses or ceiling joists hold the outside walls from pushing outward by the rafters, so when you open the hole for the staircase having a few ties across the hole until the subfloor is installed will stop any unwanted movement. The plywood subfloor glued and nailed or screwed will also stop that movement.
Build the knee walls out of 2x6, for insulation value.
Have you had an engineer look at this?

I like all questions. It makes me think of other things. 3 Beams 1 3/4x9 1/4 1.9E under each wall. Weight unknown but I ask lbr yrd. Will insert from west side through rim joist or make them 14' and insert from crawl space. A 40' lvl beam (3-1 3/4) runs the middle of crawl space with 5 posts every 8' and I have installed the 6th post at the 12'mark. Wall is 48" solid concrete on S, W, and N side and 9' on E (basement side) New idea since hearing from joining HRT-insert Mono trusses (12'long on S side of room and 16'long on N side) from N back yd side of roof (cut 13 narrow openings). Not sure of seat cut-will measure. I plan to have an engineer look at this after I get input from you guys. We had the house built 14yrs ago. I have the time and want to improve/insulate etc the house. Existing electrical and htg/air can handle the room addition. We live in E Central IL. THANKS AGAIN for the input. I have already changed the elect, ducts, & water lines in crawl space to make room for the beams.

An engineer should look at the whole thing from the start, from the weight of the structure to the size of the footing in the crawspace. If the beam is going below the floor joist, three peices are good, the engineer will tell you how many bolts or nails he wants in them. If they are going in between the joists one piece beam would be a better choice, 14ft from each side. The engineer should size the beam too. With a little inginuity the beams go in fairly easy.
I asked about seat cut because I thought you were going to install rafters but installing what we call bonus room trusses would be a better idea. I would remove all the sheeting on one side of the house and have the truss company with the crane drop the new ones in. Roof back in place and water tight in 2 days.